AP Exclusive: A look at 9/11 memorial waterfalls

NEW YORK 
The waterfalls meant to evoke memories of the Sept. 11 dead stand three stories high in a field in Brooklyn, spilling down into a pool. Four large American flags poke through the grass to mark off each corner of a World Trade Center tower footprint.

For more than three weeks, builders of the Sept. 11 memorial have been tinkering with details like water flow and year-round heating with a mock-up of the cascades, part of Michael Arad's "Reflecting Absence" memorial at ground zero.

The architect saw the waterfalls - which will empty into huge reflecting pools set above the spots where the towers once stood - tested for the first time Friday as they were demonstrated for The Associated Press.

"One of the things I wanted the water and the design to do is to mark this continuous sense of absence," Arad said. "These voids, even though water falls into them ... they never fill up, they always remain empty, and that was very important to me," he added.

The waterfall-filled pools are the centerpiece of a memorial plaza that will take up half of the 16-acre site at ground zero. The pools will be surrounded by hundreds of sweetgum and white oak trees on a cobblestoned plaza; a memorial museum is being built below ground that will open about a year after the memorial.

The foundation overseeing the memorial raised $350 million privately to build it; the government agency that owns ground zero is spending hundreds of millions more on infrastructure at the site. Two of five planned skyscrapers and a multibillion-dollar transit hub are also under construction.

The waterfall mock-up being tested in Brooklyn is 40 feet wide, representing a corner of two walls of one pool - a fraction of its actual 176-foot perimeter. The waterfalls will drop 30 feet and then another 16 feet into a center void. Each will pump 26,000 gallons of water per minute.

Visitors on Friday looked up at the waterfalls. Once the memorials open at ground zero in lower Manhattan by the 10th anniversary of the 2001 attack, visitors will look down on them from street level. A low wall covered by bronze panels inscribed with nearly 3,000 victims' names will surround the pools.

Both the waterfalls and the name panels will be backlit at night, giving them a glow that will reinforce "the memorial is about the absence of the 2,982 people that were lost," said Joe Daniels, who heads the foundation overseeing the memorial.

The mock-up was created primarily to test the weir, a small dam with fingerlike structures that helps create a rope-like water flow. It's where the water falls from the plaza level down to the base of the pool.

"The testing was required so that just the right veil of water ... is exactly what the designers are hoping to achieve," Daniels said.

The weir, made of darkened stainless steel, will be installed at ground zero within a few months after the rest of the mock-up is dismantled next week.